Thunderbird was last updated 2017 iirc??????
And I use Snow Leopard. I set up iCal mixing some tutorials on the internet a few years back, but using these will (hopefully) work! Just remember to use an app specific password

(i used p01-caldav.icloud.com as my server)
discussions.apple.com
https://nico-beuermann.de/s9y/index.php?archives/115-English.html (I don't speak german, but using google translate i was able to understand it)
https://discussions.apple.com/thread/3392474?sortBy=rank (this probably is the one i got it from, lol)
[also, sorry wowfunhappy for derailing this thread into iCloud setup territory!!! ><"]